E120 - Biodiversity II: Effects of Plant Biodiversity on Population and Ecosystem Processes

Introduction

This experiment (often called the "Big" Biodiversity Experiment; the "small" experiment is no longer maintained) determines effects of plant species numbers and functional traits on community and ecosystem dynamics and functioning. It manipulates the number of plant species in 168 plots, each 9 m x 9 m, by imposing plant species numbers of 1, 2, 4, 8, or 16 perennial grassland species. The species planted in a plot were randomly chosen from a pool of 18 species (4 species, each, of C4 grasses, C3 grasses, legumes, non-legume forbs; 2 species of woody plants). Its high replication (about 35 plots at each level of diversity) and large plots allow observation of responses of herbivorous, parasitoid and predator insects and allow additional treatments to be nested within plots. Planted in 1994, it has been annually sampled since 1996 for plant aboveground biomass and plant species abundances and for insect diversity and species abundances. Root mass, soil nitrate, light interception, biomass of invading plant species, and C and N levels in soils, roots, and aboveground biomass have been determined periodically. In addition, soil microbial processes and abundances of mycorrhizal fungi, soil bacteria and other fungi, N mineralization rates, patterns of N uptake by various species, and invading plant species, have been periodically measured in subprojects in the Biodiversity Experiment.

Key Results

Using two long-term (>12 years) biodiversity experiments, we show that the effects of diversity on biomass productivity increased and became less saturating over time. Our analyses suggest that effects of diversity-dependent ecosystem feedbacks and interspecific complementarity accumulate over time, causing high-diversity species combinations that appeared functionally redundant during early years to become more functionally unique through time. Consequently, simplification of diverse ecosystems will likely have greater negative impacts on ecosystem functioning than has been suggested by short-term experiments. Reich et al. 2012 Science

Results of 11 experiments show that ecologically relevant decreases in grassland plant diversity influenced productivity at least as much as ecologically relevant changes in nitrogen, water, CO2, herbivores, drought, or fire. In particular, a change in plant diversity from four to 16 species caused as large an increase in productivity as addition of 54 kg•ha−1•y−1 of fertilizer N, and was as influential as removing a dominant herbivore, a major natural drought, water addition, and fire suppression. Our results suggest that the loss of biodiversity may have at least as great an impact on ecosystem functioning as other anthropogenic drivers of environmental change, and that use of diverse mixtures of species may be as effective in increasing productivity of some biomass crops as fertilization and may better provide ecosystem services. Tilman et al. 2012 PNAS

Greater plant species numbers led to greater ecosystem stability (lower year-to-year variation in total plant biomass) but to lower species stability (greater year-to-year variation in abundances of individual species), with the stabilizing effect of diversity mainly attributable to statistical averaging effects and overyielding effects (Fig 4; Tilman et al, submitted).

Data gathered this past field season shows that soil total C has now become an increasing function of plant species numbers (Fig 5).

Our results have helped resolve a debate about why plant diversity affects ecosystem functioning. Such resolution was accomplished by a Paris symposium in which we made CDR biodiversity data available so others could test their alternative hypotheses; by a paper by 12 ecologists with divergent views that explored areas of agreement and articulated areas in need of 10 further testing (Loreau et al. 2001); and by our analyses of alternative hypotheses using results of the CDR biodiversity experiment (Tilman et al. 2001b).

Future Research

This experiment, now in its 17th year, is still highly productive, resulting in 43 peer reviewed papers during the current grant. We propose continuing our annual sampling of primary productivity and plant species abundances, and periodic sampling of soil properties, heterotroph communities, and other parameters. Among the questions that we will address in future research are:

(1) Why did the impact of plant diversity on primary productivity increase so much from 1996 to 2005 and then slow during the past 5 years? Might productivity in low diversity plots slowly increase, and eventually catch up with that of high diversity plots?

(2) What are the long-term prospects for high diversity plantings to serve as C sinks, and what role might herbivory play, since many herbivores preferentially feed on legumes? Can high diversity plant mixtures accrue more soil C and N than occurs in nearby native grassland ecosystems?

(3) What are the quantitative dependences of various ecosystem services on plant species composition and diversity? We will explicitly quantify such services as improved ground water quality, provisioning of predatory and parasitoid insects of importance for the control of agricultural pests, provisioning of pollinators of agricultural relevance, reductions in the diversity and prevalence of agriculturally important pathogens, and the ability of high diversity buffer strips to capture sediment in surface runoff and remove excess nutrients and other agrichemicals. We also estimate the economic values of these and other ecosystem services.

(4) Finally, we note that the CDR Biodiversity Experiment, the oldest and best replicated large-plot biodiversity experiment in the world, has become an important resource for many non-CDR researchers who visit CDR to sample the experiment to address their own novel questions (Zak et al. 2003, Haddad et al. 2009, 2011; P Vanderkoornhuyse, Universite de Rennes). We actively encourage and continually seek such outside collaborations and independent research initiatives.